1. Technical Field
The invention relates to the field of attenuators, and more particularly to radio frequency attenuators.
2. Background Art
Interference Canceller Systems (ICS) have been produced for use on military platforms. This kind of system, also known as a “phase canceller,” operates by obtaining a radio-frequency (RF) sample of an interfering source and summing it, precisely weighted in magnitude and phase, into the received path to cancel the received interference, leaving the signal(s) of interest.
A fundamental block or element in the circuitry in this system is a vector modulator, also known as a “weight circuit.” The weight circuit uses several networks each consisting of positive-intrinsic-negative (PIN) attenuator diodes, couplers, resistors and nonlinear DC drive circuitry to form a voltage variable resistance element capable of handling moderate RF input power levels. The resistance element creates a reflected signal that continuously covers the range of in-phase (positive reflection coefficient), inverted (negative reflection coefficient) or zero. A PIN diode is a current controlled device, in which the RF resistance is a function of the forward DC current PIN diodes are reasonably low in RF distortion production when driven with moderate to high DC currents, or when subjected to low RF currents. But they can have considerate RF nonlinearities and create distortion when subjected to moderate to high RF currents and low DC control currents.
For use in an ICS system, the control law of the weight circuit element must provide at its output reasonably linear control scaling of the applied RF voltage relative to the control voltages, while sequentially driving the branches of diodes and resistors so that they remain within conditions of applied RF and DC that result in acceptable output distortion levels.
The driving circuit of the weight network of existing systems (FIG. 1) uses a single power MOSFET and a chain of rectifier diodes and resistors to provide the current sequencing into the branches and to obtain an approximately linear control characteristic.
Consider a continuously rising control voltage that is to result in a variable RF impedance at the port “RF In/Out”. When the MOSFET current is cut off, all PIN diodes are in a high RF impedance state, which is coupled by the transformer to the port. As the MOSFET begins to conduct, first the PIN diodes in the first (bottom, with resistors “A”) branch begin conducting DC current and drop in RF resistance. Higher branches will not begin to conduct until the voltage at the MOSFET drain has fallen a sufficient number of diode drops below the +V supply to create a forward voltage across the PIN diodes of each branch. As MOSFET current increases further and when the resistance of the first branch becomes limited by the value of the fixed resistors (“A”), the PIN diodes in the next higher branch begin to conduct and the resistance of this second branch begins dropping. With increasing MOSFET current, the pattern continues with each branch successively conducting until the PIN diodes in the top branch are driven to their minimum RF resistance, where the total network resistance then consists of all branches in parallel at their minimum resistances. A nonlinear network provides degeneration to the source terminal of the MOSFET to approximate a linear control characteristic (of reflection coefficient versus control voltage).
A serious deficiency of such a previously known scheme is that a large portion of the DC power used by the network is consumed by the rectifier diodes and in driving earlier sequenced PIN diodes with more current than is useful to further control the PIN diodes. The drop across each diode of the network is on the order of one volt, yet the current for every PIN diode pair derives from the approximately 12V power supply. Under conditions giving the lowest network RF resistance (all PIN diodes fully conducting), only a small portion of the DC power dissipation is in the PIN diodes. As the control voltage increases, current drawn by all activated branches will continue to increase, limited eventually by the supply voltage at rather high maximum power dissipation.
In addition, the use of a MOSFET leads to possible production problems, as the turn-on voltage of MOSFETS is not well controlled from one batch or sample to anther. For large scale production, this would require part selection or inclusion of the MOSFET within a feedback loop of an operational amplifier to provide control consistency.
The circuitry of the present invention provides a means for each branch of the attenuator to draw current from a lower voltage power supply, while still providing for sequenced biasing of the successive branches and linearization of the overall control characteristic, and by using small, inexpensive bipolar transistors to provide improved repeatability in manufacture.
While the above cited references introduce and disclose a number of noteworthy advances and technological improvements within the art, none completely fulfills the specific objectives achieved by this invention.